1. Field of the Invention
The present invention relates to a fluorescence imaging apparatus for taking a fluorescent image by using excitation light and a light source unit for the fluorescence imaging apparatus.
2. Description Related to the Prior Art
In fields of biochemical and molecular biological researches, fluorescence imaging apparatuses for sample analysis are known (see U.S. Pat. No. 8,658,989 (corresponding to Japanese Patent No. 5323130) and Japanese Patent Laid-Open Publication No. 2010-091456). The fluorescence imaging apparatus applies excitation light to a sample containing a fluorescent substance (or fluorescent material) and detects excited fluorescence to take a fluorescent image. The sample is, for example, a biological substance (e.g. DNA, RNA, enzyme, antibody, or antigen) or a living body of a mouse, a rat, or the like.
In a case where a biological substance is used as a sample, a fluorescent dye is added as a marker substance (marker) to the sample. The fluorescent dye generates fluorescence by the application of the excitation light. The fluorescent image is obtained by detecting the fluorescence from the fluorescent dye. The fluorescent image shows reaction and distribution of the biological substance in the sample. By analyzing the fluorescent image, isolation or identification of a gene order, expression levels of genes, or protein is performed. Ina case where a living body (or living organism) is used as a sample, the excitation light is applied to the living body to take the fluorescent image. The living body has tissue containing a fluorescent substance that generates autofluorescence, or a fluorescent dye may be given as a marker to the living body. The living body is examined by analyzing the distribution of the autofluorescence or the fluorescent dye in the fluorescent image of the living body.
The fluorescence imaging apparatus comprises a light source unit and a camera section. The light source unit applies excitation light to a sample. The camera section has an image sensor that takes fluorescent images. Different fluorescent substances have different excitation wavelength ranges. A type of excitation light to be emitted from the light source unit is selected based on the excitation wavelength range of the fluorescent substance. The image sensor detects the fluorescence excited by the selected excitation light. The camera section is provided with a filter unit. For example, the filter unit allows only the fluorescence from the fluorescent substance to enter the image sensor. The light source unit described in the U.S. Pat. No. 8,658,989 comprises different types of emission units that apply different colors of excitation light. The different types of emission units have different types of light emitting elements that emit different colors of light, respectively. The light emitting element is an LED (Light Emitting Diode), for example. The light source unit of the U.S. Pat. No. 8,658,989 selectively uses the emission units of different colors. Thereby, the light source unit is easily capable of exciting different types of fluorescent substances with different excitation wavelength ranges.
The Japanese Patent Laid-Open Publication No. 2010-091456 describes a light source unit comprising three types of emission units that emit excitation light of three colors in a blue (B) region, a green (G) region, and a red (R) region. A “B emission unit” has a B-LED that emits blue light and a “B excitation light filter” placed in front of the B-LED. A “G emission unit” has a G-LED that emits green light and a “G excitation light filter” placed in front of the G-LED. An “R emission unit” has an R-LED that emits red light and an “R excitation light filter” placed in front of the R-LED.
The excitation wavelength range of the fluorescent substance is narrower than the emission wavelength range of the LED of each color. The excitation light filter of each color cuts wavelengths of light from the LED of each color outside the excitation wavelength range. Thereby the wavelength range of the light to be applied to the fluorescent substance is narrowed.
A light component outside the excitation wavelength range applied to the fluorescent substance does not contribute to the excitation of the fluorescent substance, and what's worse, increases the brightness level of the background in the fluorescent image. As a result, the contrast between the fluorescence and the background is reduced. Since the excitation light filter removes the light component outside the excitation wavelength range, the increase in the brightness level of the background is suppressed and excellent distinguishability of the fluorescence in the fluorescent image is ensured.
To reduce the cost of the light source unit, the inventor is examining to mount different types of LEDs (B-LED, G-LED, R-LED, and the like) in one-dimensional or two-dimensional arrangement on a single circuit board. In other words, the single circuit board is shared by the emission units of different colors. A front cover is attached to the front face of the circuit board. For example, the front cover is also shared by the emission units of different colors. The front cover is provided with emission windows through which different colors (B, G, and R) of excitation light is applied. The emission window of each color is composed of the excitation light filter of the corresponding color (B, G, or R) and a window frame into which the excitation light filter is fit.
In a case where the single circuit board is shared by the different types of emission units, the circuit board causes harmful light that is harmful to the fluorescent image. The harmful light is imaged and appears in the fluorescent image. To be more specific, the circuit board is a printed circuit board in which a wiring pattern and the like are printed on a substrate. For example, resin (e.g. glass epoxy) with light transmitting property is used as the substrate. A fluorescent component contained the resin (glass epoxy) is excited by the blue light from the B-LED, and thereby generates green fluorescence (resin-derived fluorescence) derived from the resin. The green resin-derived fluorescence is imaged and appears as the harmful light in the fluorescent image.
In the B emission unit, the B excitation light filter is placed in front of the B-LED. Even if the green resin-derived fluorescence occurs, the green resin-derived fluorescence is cut by the B excitation light filter and does not go out of the B excitation light filter. However, the circuit board has the light transmitting property. In a case where the single circuit board is shared by the different types of emission units, the green resin-derived fluorescence is transmitted to, for example, the G emission unit adjacent to the B emission unit. In a case where the wavelengths of the green resin-derived fluorescence are included in the transmission wavelength range of the G excitation light filter, the green resin-derived fluorescence is released through the G excitation light filter.
In a case where the fluorescent substance excited by the B excitation light generates the fluorescence of a green region, the wavelengths of both the green resin-derived fluorescence and the green fluorescence from the fluorescent substance are within the same green region. For example, in a case where the filter unit of the camera section passes both the green resin-derived fluorescence and the green fluorescence from the fluorescent substance, both the green fluorescences enter the image sensor 22. Thus, the resin-derived fluorescence is imaged and appears in the fluorescent image. In the fluorescent image, the green resin-derived fluorescence increases the brightness level of the background and reduces the contrast between the background and the green fluorescence from the fluorescent substance. Thus, the green resin-derived fluorescence is harmful to the fluorescent image.